I think it is both environment and birth. Someone can be born with their brain wired a certain way so that they natural learn things easier and more effectively. We are still looking for the gene that dictates intelligence and seperates us from other primates. We have over 99% of our DNA in common with them but it is that small amount that makes us intelligent. So it appears if you can be born with a gene that dictates intelligence, why wouldn't it come in degrees?